1. Field of the Invention
The present invention relates to solid-state image pickup devices used for digital still cameras, digital video cameras, or the like.
2. Description of the Related Art
In typical solid-state image pickup devices, electric signals that are generated through photoelectric conversion performed in a plurality of pixels arranged in a two-dimensional matrix are transmitted to a signal processing circuit in the subsequent stage via amplifiers provided in the respective columns. In recent years, there has been a demand for solid-state image pickup devices to have more pixels and to operate at higher speed, so that the bandwidth set in each amplifier has tended to be wide. Such an increase in signal bandwidth leads to an increase in the integral value of random noise, which is proportional to the signal bandwidth of the amplifier. Japanese Patent Laid-Open No. 2008-78894 describes a configuration in which a capacitor is connected to an amplifier when clamping the reference potential of the amplifier. As a result of clamping the reference potential of the amplifier by connecting a capacitor to the output end of the amplifier, the bandwidth of the amplifier is limited. Accordingly, random noise generated from the amplifier is reduced.
In a solid-state image pickup device having a plurality of amplifiers, for example, amplifiers provided in individual columns, the amplification factor of the amplifiers may be changed in various ways in accordance with the intended application. For example, driving for capturing an image having a wide dynamic range in which a dark subject and a light subject exist may be performed. Also, driving for securing a dynamic range in each of an addition mode and a non-addition mode, and driving for changing the amplification factor of signals in individual pixels of individual colors may be performed. In such driving operations, the amplification factors of amplifiers range from minus several dB to several tens of dB. However, in the case of changing the amplification factors of amplifiers in accordance with an image capturing condition, the driving method according to Japanese Patent Laid-Open No. 2008-78894 does not always sufficiently reduce random noise generated from the amplifiers. This is because, in the method described in Japanese Patent Laid-Open No. 2008-78894, a frequency band is limited by a feedback capacitor that determines an amplification factor of an amplifier and a signal storage capacitor connected to an output end of the amplifier. For this reason, the frequency band of the amplifier becomes wider as the amplification factor decreases with a small feedback capacitance, and random noise increases in proportion to the frequency bandwidth. In other words, random noise generated from the amplifier is not always sufficiently reduced depending on an amplification factor that is set.